Grazing impact of calanoid copepods on phytoplankton size-fractions in Jangmok Bay, south coast of Korea

Abstract

Grazing impacts of calanoid copepods on size-fractionated phytoplankton biomass (chl-a) were measured in the Jangmok Bay of Geoje Island monthly from November 2004 to October 2005. The ingestion rate of calanoid copepods on total phytoplankton biomass ranged between >1 and 215 ng chl/copepod/day during bottle incubations. In grazing experiments on 3 phytoplankton size categories (<3, 3-20 and >20 ㎛), our result indicated microphytoplankton (>20 ㎛) was the primary food source for calanoid copepods. The ingestion rate on microphytoplankton showed a significant increase(r=0.93, p<0.01) with chl-a concentration. Nanophytoplankton (3-20 ㎛) showed negative ingestion rate from June to October 2005 because calanoid copepods consumed their microzooplanktic predators(specially, ciliates). Calanoid copepods were unable to feed efficiently on picophytoplankton(<3 ㎛) due to unfavourable size. Between >0.1-27.7% (average 3.6±15.8%) of phytoplankton biomass were removed daily by calanoid copepods during grazing experiments. High grazing pressure was found in January (27.7%), February (27.4%) and March (15.6%) 2005, but values in other months were low (<5%). Our result suggests calanoid copepods play an important role in controlling the biomass and size structure of phytoplankton in winter and early spring.